In a conventional electromagnetic relay, two stationary terminals having a stationary contact are positioned and fixed, and an electric circuit is opened or closed by moving a movable element having a movable contact such that the movable contact and the stationary contact come into or out of contact with each other.
An electromagnetic repulsion force (hereinafter, this electromagnetic force is referred to as a contact point electromagnetic repulsion force) is generated at a contact point of the movable contact and the stationary contact by an electric current flowing in a reverse direction in a part where the movable contact and the stationary contact face each other. The contact point electromagnetic repulsion force works such that the movable contact and the stationary contact are separated from each other.
As shown in FIG. 8, a movable yoke 93 and a stationary yoke 94 is attached to a shaft 92 which is integrated with a movable core 90 and attached to a movable element 91, the movable element 91 being interposed between the movable yoke 93 and the stationary yoke 94, and a yoke attraction force is generated between the movable yoke 93 and the stationary yoke 94 by a magnetic flux flowing in the movable yoke 93 and the stationary yoke 94 when the movable element 91 and a stationary terminal 95 contact to each other. The movable yoke 93 pushes the movable element 91 to the stationary terminal 95 so as to limit a separation of the contacts caused by the contact point electromagnetic repulsion force (for example, refer to Patent Document 1).
The movable yoke 93 is slidably attached to the shaft 92, and the stationary yoke 94 is fixed to the shaft 92. The movable element 91 and the movable yoke 93 are urged toward the stationary yoke 94 and the stationary terminal 95 by a pressure contact spring 96 attached to the shaft 92. The movable core 90 is attracted toward a stationary core 98 by an electromagnetic attraction force generated when the excitation coil 97 is energized.
When only the yoke attraction force is increased without increasing the electromagnetic attraction force in order to limit the separation caused by the contact point electromagnetic repulsion force even in a short circuit in which large amount of electric current flows, a phenomenon described below may occur.
After the movable yoke 93 is pushed the movable element 91 to the stationary terminal 95 (a situation shown in FIG. 8), the stationary yoke 94 is attracted toward the movable yoke 93 by the yoke attraction force, and the movable core 90 and the shaft 92 integrated with the stationary yoke 94 are urged in a direction opposite from the attraction force caused by the electromagnetic attraction force.
When the yoke attraction force is larger than the electromagnetic force, the movable core 90 moves apart from the stationary core 98, and accordingly the electromagnetic attraction force decreases. As a result, the contacts may be separated from each other.
When the electromagnetic force is also increased and set to be larger than the yoke attraction force, the above-described phenomenon does not occur. However, in this case, the excitation coil 97 may be large in size in order to increase the electromagnetic attraction force.
On the other hand, Patent Document 2 discloses an electromagnetic relay in which a stationary yoke is fixed to a casing. According to this, since the stationary yoke is immovable, the stationary yoke is not attracted toward the movable yoke by a yoke attraction force after the movable yoke pushes the movable element to a stationary terminal, and accordingly a separation of contacts can be limited.
However, a contact room in which the movable element and the stationary terminal are provided becomes high temperature by a heat generated by an electric current during energization or an electric arc during not energization. Since the stationary yoke is located in the contact room in the conventional electromagnetic relay disclosed in Patent Document 2, a contact point of the stationary yoke and the casing may become high temperature, and accordingly a boning power may be likely to be insufficient when the stationary yoke and the casing are bonded by a bonding agent or an adhesion tape.
Accordingly, brazing or welding is selected as a method for bonding the stationary yoke and the casing, but in this case, a positioning of the stationary yoke to the casing may be not easy.
Moreover, a resin material used for securing insulation may not resist heat of brazing.